1. Field of the Invention
This invention relates to pumps and more particularly to airless paint spray pumps having bypass valves.
2. Prior Art
Diaphragm airless paint spray pumps utilizing a pumping chamber subdivided into a driving fluid chamber and a driven fluid chamber by a diaphragm, with the driving fluid being alternately pressured and relieved by a reciprocating piston are well known to the art. See, for example, U.S. Pat. Nos. 3,254,845 to Schlosser and RE 29,055 to Wagner. Such airless diaphragm pumps have become very popular. A common feature of such pumps is the provision of a bypass valve in the pumped fluid line. The pumped fluid side of the pump includes an inlet having a check valve in it and an outlet having a check valve in it such that flow to and from the pumped fluid subchamber is one way. In order to allow priming of the pump, a low pressure bypass is provided which is valve controlled. With the bypass valve open pumped fluid from the pumped fluid subchamber is released at low pressure usually to a conduit to the supply source.
The bypass valve allows the pump to be primed by providing for relatively pressure free circulation of the pumped fluid through the pumping chamber.
Since each airless diaphragm paint spray pumps frequently operate at pressures between 2,000 and 3,500 PSI, actuation of the bypass valve to allow priming is important. Additionally, however, because of the high pressures involved, when the prime mover is turned off, it would be beneficial to provide a system for allowing the high pressure remaining in the pump and in the line to the pumped fluid utilizer, i.e. the spray gun, to be bled off automatically. One such system has been previously proposed which relies upon an hydraulic system comparing the pressure on the driven fluid side of the diaphragm to the pressure on the driving fluid side of the diaphragm. When the driven fluid pressure substantially exceeds the driving fluid pressure, a bypass is open. See, for example, U.S. Pat. No. 4,022,381 to Karliner. While such a system is effective for bleeding high pressure in the pump after the prime mover has been throttled or turned off, the system does not provide for actuation when the driving fluid is maintained at a relatively high pressure, such as is the case when the motor is full running but a low driven pressure is required for priming.
It would also be desirable to provide an automatic pressure limiter on the driven fluid side. Prior airless diaphragm pumps normally utilize a pressure release valve on the driving fluid side for porting the hydraulic driving fluid from the driving fluid subchamber back to the reservoir. Maximum pressure output of the system is thus controlled by the driving fluid side pressure release valve. However, since such valves can conceivably malfunction, it would be a desired feature to provide such a pressure release on the driven fluid side. Although such high pressure release valves have previously existed in some embodiments, they have not been successfully integrated into commercial diaphragm spray pumps which continue to rely upon the hydraulic side pressure release valve.
It would also be an advantageous contribution to the art of all of the above desired features could be incorporated into a single device which functioned as a combination priming valve, high pressure bleed valve and pressure limiter. It would be further advantageous if the device functioned automatically upon termination of the prime mover and selectively at the operator's desire.